Cook the sliced potatoes in a large pan of boiling water for 5-10 minutes until just tender. Drain and set aside.
Meanwhile, heat a 22cm ovenproof frying pan with a lid and cook the bacon pieces for 5 minutes until starting to brown. Discard the excess fat and set the bacon aside.
Put the beaten eggs, milk, spinach, cooked potato and bacon in a large bowl and stir to combine.
Pour the frittata mixture into the frying pan. Cover with the lid and cook over a medium-low heat for about 10 minutes – as it steams, the spinach will wilt into the egg mixture. Preheat the grill to medium.
Remove from the heat and scatter the grated cheddar over the top. Put the pan under the grill and cook for 10 minutes until set and golden. Serve with the salad.